486 Motherboard POSTs then freezes \ VOGONS

486 Motherboard POSTs then freezes

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First post, by thevdm

Posted on 2021-03-06, 11:39

thevdm Offline

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Location: Kent, UK

Hi,

I have a 486 motherboard that posts successfully and freezes shortly after handover.

Model: DT-Tornado2 / Jac-Up3

POST will complete, DOS will start to load from a floppy and it will freeze. I don't think it's related to DOS or the floppy drive etc... as the same happens when I enter the BIOS setup screen, I can get one or two key presses in then the system freezes.

Tests have been done with and without expansion cards installed, I have tried combinations of:

With/Without Cache installed (and jumpers set accordingly)
Different RAM (known working from my other 486 desktop)
Different power supply (known working from other 486)
486 SX25, SX2 50, DX2 66, DX4 OD 100 (onboard regulators). All bar the DX4 OD are known working Jumpers set accordingly
Onboard IDE enabled/disabled

Physically the board is in very good condition, the tantalum caps visually look to be good, traces/solder joins visually look ok.

I have a POST card being delivered today to see if that brings up anything useful.

What would be your next area to test?

Edit: It might be worth adding that shorting the reset pins still works when the motherboard freezes, I'm not sure if this will indicate a "different type of freeze". I have also reseated all socketed chips.

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DT-Tornado 2 / Jac-Up3 motherboard

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Gaming rig: Dell Dimension XPS T500 - PIII 500 - 288MB RAM - Voodoo3 3000 - SoundBlaster Live! Value - DVD-ROM - CD-RW - 3.5" 1.44 - 98SE & 2000 dual boot
A nostalgic pile of laptops from the late 80's to late 90s.

Reply 1 of 6, by gnif

Posted on 2021-03-06, 14:29

gnif Offline

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Bad or misconfigured L2 SRAM?
Also, lots of tantalum caps on there that may need replacing

Reply 2 of 6, by thevdm

Posted on 2021-03-06, 14:50

thevdm Offline

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Location: Kent, UK

I've tried with the cache disabled and removed, this gives the same results (just to confirm L2 SRAM and Cache are the same?).

The tantalum caps could be the cause, although they visually look good, visually and electrically good are very different. I guess it might be worth noting all the capacitor values and ordering some modern replacements, I have little to loose otherwise.

Reply 3 of 6, by thevdm

Posted on 2021-03-09, 18:48

thevdm Offline

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Location: Kent, UK

Update: I found another Jac-Up3 board on ebay which arrived today and works perfectly, I've also ordered some replacement capacitors as if I can fix the original board it wouldn't hurt to have a spare.

This 486 build is getting to be an expensive one, but it being the same make/model as my very first PC it's more of an exercise of sentiment than value vintage computing.

Reply 4 of 6, by majestyk

Posted on 2021-03-09, 19:33

majestyk Offline

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These tantalum capacitors don´t age like regular electrolytics. They are either still perfectly o.k. or - sometimes - shorted. In this case the psu won´t start up .
I don´t think replacing all tantalum capacitors will be helpful.
Did you swap BIOSes between the two boards or swap the keyboard controllers?
Video RAM can also be an issue. You could try with an ISA VGA card instead.

Reply 5 of 6, by thevdm

Posted on 2021-03-09, 19:57

thevdm Offline

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Location: Kent, UK

I've not tried switching the BIOS or keyboard controller yet, that is in the plans for this weekend when I have more time. It is possible that either of these are the cause of the issue. If it's the BIOS I'll have to get hold of an EEPROM programmer, it would be nice to have a backup anyway as I can't find a BIN file for it online (or any BIOS chips on their own) - This is partly why I bought the second board.

If the keyboard controller is the cause of the issue it should be a cheap fix - I can't find any of the exact model number, but with some research there's probably a few other models that are direct replacements.

Unfortunately I don't have an ISA graphics card, but acquiring one shouldn't be a problem, I'll try the BIOS/Keyboard controller first as I have a known working chips to test with. If video memory is the issue, the chips are DIP (not socketed), so should be easily replaceable if I can find replacements.

Thank you for your help.

Reply 6 of 6, by thevdm

Posted on 2021-03-12, 21:22

thevdm Offline

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Posts: 41
Joined: 2018-03-12, 18:48
Location: Kent, UK

Final update on this one, switched the keyboard controller chips and the board works perfectly. Very happy it's the cheapest/easiest component to switch.

Thank you all for your help.

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